Transducer head assembly and apparatus for using same in a plural- track record disk system

ABSTRACT

Described is a transducer head assembly for magnetic disk files, the assembly using the informational data itself recorded on the disk in the form of a plurality of annular tracks for precisely positioning the head assembly with respect to the individual tracks. For this purpose, the assembly includes an upper head cooperable with the upper face of the record disk and a lower head cooperable with the lower face of the record disk, the lower head being spaced from the upper head one-half the distance between a pair of adjacent tracks such that one head may be used for positioning purposes by centering same between two adjacent tracks on the respective face of the disk, while the other head being thereby centered with a track on its respective face of the disk, may be used for recording, reproducing or erasing data with respect to the later track. 
     Also described is a scheme of recording the data at different clock frequencies on different tracks, such that the clock frequencies between adjacent tracks differ by at least 2%, with the maximum difference between the clock frequency of any particular track and the nominal clock frequency being less than 5%.

BACKGROUND OF THE INVENTION

The present invention relates to a novel transducer head assembly foruse with a multi-track record disk. The invention also relates to amethod and apparatus using the novel transducer head assembly forrecording, reproducing or erasing data with respect to the record disk,and further, to a method of using such assembly for initializing avirgin record disk.

Record disks are commonly used for storing data in computers and otherdata processing systems. Such record disks are provided on both faceswith a layer of a magnetizable material for recording the data in theform of a plurality of spaced, annular tracks. Some disk systems, ordisk files as they are frequently called, include a plurality of suchrecord disks stacked on a common spindle and driven by a common drive,whereas others include only one record disk. Because of the expenseinvolved in providing a head for each track, most systems use one headper disk, which means that the head must be very accurately positionedto align itself with the selected track with respect to which the datais to be recorded, reproduced, or erased.

In order to obtain the maximum storage capacity per disk, a large numberof tracks are provided on each surface, for example 200 tracks per inchor more. A factor limiting the track density, however, is the precisionwith which a head can be positioned with respect to a selected track.Positional inaccuracies may arise from a number of factors, includingspindle eccentricity; temperature-coefficient variations with respect tothe disk, base, head and transducer; nonlinearity in the transducer; andsetting-up errors. If the head is not precisely positioned on a trackfrom which recorded data is to be read, the signal level will be reducedand errors may therefore occur.

A number of techniques have been devised for reducing such positionalinaccuracies. One system utilizes an arrangement which generates apositional wave-form (e.g., sine-wave) as the head moves towards thespindle, the zero crossings defining the track positions. Such a system,however, does not take into account effects external to the disk, suchas dimensional variations caused by unequal temperature-coefficients.Another technique utilizes "dedicated tracks", in which the head isaligned with data prerecorded on special tracks on the record disk.These systems, however, require that a substantial recording surface beallocated to the dedicated tracks and therefore deprive such surfacesfrom being used for recording the informational data. They are thereforeused mostly with multi-disk systems in which one disk includes theprerecorded positional data.

SUMMARY OF THE PRESENT INVENTION

The present invention provides an arrangement which permits the use ofthe maximum storage capacity of the disk for recording informationaldata, by enabling very precise positioning of the transducer head withrespect to the tracks on the disk while at the same time utilizing thecomplete surfaces of the disk for recording the informational data.

According to one aspect of the present invention, the novel transducerhead assembly comprises a support, an upper transducer head carried bythe support and cooperable with the upper face of the record disk, and alower transducer head carried by the support and cooperable with thelower face of the record disk. The lower transducer head is spaced fromthe upper head one-half the distance between a pair of adjacent tracks,such that one head cooperable with one face of the record disk may beused for positioning purposes by exactly centering the head between twoadjacent tracks on one face of the record disk, while the other head isthereby centred with a track on the other face of the record disk andmay therefore be used for recording, reproducing or erasing data withrespect thereto.

The foregoing arrangement, in which one head is used for positioningwhile the other head is used for recording or reading, enables all theavailable storage space on both faces of the record disk to be utilizedfor recording the informational data, the informational data itselfbeing used for precisely positioning the transducer head when recording,reproducing or erasing the data from the individual tracks.

The invention is described below with respect to magnetic transducerheads for use with magnetic record disks, this being a preferredembodiment.

According to another aspect, the invention also provides a method ofrecording, reproducing, or erasing data from a record disk by the use ofthe above-described transducer head assembly. The novel method comprisesthe steps of positioning one transducer head of the head assemblyexactly between two adjacent tracks on one face of the record disk, andusing the other transducer head for recording, reproducing or erasingdata with respect to the track with which it is centered on the outerface of the record disk.

According to another feature, the informational data is recorded atdifferent clock frequencies on adjacent tracks, the data of differentclock frequencies of each pair of adjacent tracks on one surface of therecord disk being filtered, separated, and used to center the one headexactly between the pair of adjacent tracks, and thereby to center theother head with a track on the other face of the record disk.

According to a further feature of the invention, there is provided amethod of recording data at different clock frequencies on aplural-track record disk, characterized in grouping the tracks into aplurality of groups, recording the data at different clock frequenciesamong the tracks in each group with the different clock frequenciesbeing interlaced between adjacent tracks such that each odd track has arelatively large clock frequency difference with respect to the adjacenteven track but a relatively small clock frequency difference withrespect to the next odd track, the clock frequency at which the data isrecorded being the same at the corresponding tracks in the differentgroups, each group of tracks including two sub-groups; and reversing theclock frequency difference in each sub-group so that the clock frequencydecreases with respect to a nominal clock frequency in one sub-group andincreases with respect to the nominal clock frequency in the othersub-group.

The invention also provides a system for recording, reproducing, orerasing data with respect to a plural-track record disk, the systemincluding the foregoing transducer head assembly; a drive for drivingthe assembly with respect to the record disk; position control meanscomparing the amplitudes of the signals from adjacent tracks read bysaid one data head used for positioning purposes and controlling thedrive so that said signal amplitudes are exactly equal; and meanscontrolling said other head to record, reproduce, or erase data withrespect to the track on the other face of the record disk with which itis centered.

According to a further feature, the system further includes meansrecording the data on adjacent tracks at different clock frequencies,and filters separating the different frequencies read by the positioninghead before the amplitudes are compared and used for controlling thetransducer head assembly drive.

The invention also provides a novel method for initializing a virginrecord disk by the use of the foregoing transducer head assembly.

Further features and advantages of the invention will be apparent fromthe description below.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is herein described, by way of example only, with respectto a preferred embodiment thereof illustrated in the accompanyingdrawings, wherein:

FIG. 1 is a schematic illustration of a magnetic transducer headassembly constructed in accordance with the invention, and a single diskrecording system using such head assembly;

FIG. 2, 2A, 2B schematically illustrates the positioning, with respectto the track on one face of the record disk, of one of the heads in thetransducer head assembly of FIG. 1;

FIG. 3 schematically illustrates the positioning of the other head inthe assembly with respect to the tracks on the other face of themagnetic disk;

FIG. 4 is a diagram of the different clock frequencies at which data isrecorded in the various tracks of the magnetic disk; and

FIG. 5 is a diagram schematically illustrating the overall system forrecording and reproducing data with respect to the multi-track disk, andalso for initializing a virgin record disk.

DESCRIPTION OF THE PREFERRED EMBODIMENT General Arrangement

The recording system illustrated in FIG. 1 includes a single magneticdisk 2 mounted on a spindle 4 rotated by a disk drive 6. Both faces 2a,2b of the record disk are provided, by an initializing procedure such asone described below, with a plurality of spaced annular tracks withrespect to which data is to be recorded, reproduced or erased by meansof a transducer head assembly, generally designated 8. This headassembly includes a support 10 carrying an upper magnetic transducerhead 12 cooperable with the upper face 2a of the record disk, and alower transducer head 14 cooperable with the lower face 2b of themagnetic disk.

The transducer head assembly 8 is positioned with respect to record disk2 by means of a drive, generally designated 16. The latter drive movesthe head assembly radially with respect to spindle 4, to a selectedtrack as specified by an address supplied from an input device. Theinput device, which for the sake of simplification is represented bycentral processor block 18, supplies the data to be recorded andreceives the data to be reproduced, and also supplies the track addressat which the operation is to be performed.

Drive 16 is controlled by a system, generally designated 20, toprecisely align the head during the recording, reproducing, or erasingoperation with respect to the selected track. The data path from or tocentral processor 18 also includes system 20, as schematically shown inFIG. 1.

The disk drive 6, head position drive 16, and the manner of addressingsame from the central processor 18 (or other input device) are all wellknown and therefore for the sake of brevity they are not describedherein except when necessary to explain the present invention. Thepresent invention concerns primarily the construction of the transducerhead assembly 8 and the manner of precisely controlling its position bycontrol system 20.

Construction of Transducer Head Assembly

The lower magnetic head 14 of the transducer assembly 8 is spaced fromthe upper magnetic head 12 one-half the track pitch, i.e., the distancebetween a pair of adjacent tracks on the record disk. This is shownsomewhat exaggeratedly in FIG. 1, wherein it will be seen that the lowerhead 14 is spaced outwardly in a radial direction with respect to disk 2from the upper head 12. With such an arrangement, it will be seen thatwhen one head is exactly centered between and straddles two adjacenttracks on one face of the record disk, the other head will be exactlyaligned with a track on the other face of the record disk. Thus, onehead cooperable with one face of the disk may be used for positioningpurposes, and the other head, being thereby exactly aligned with a trackon the other face of the record disk, may be used for recording,reproducing or erasing informational data.

This is more clearly shown in FIGS. 2 and 3 which illustrate,respectively, the positions of the two heads 12, 14 with respect to thetracks on their respective faces 2a, 2b of the magnetic disk. Shown inFIG. 2 is the condition wherein the upper head 12 is exactly centeredbetween and straddles two adjacent tracks (2a-1, 2a-2) on the upper face2a of the record disk, this condition being indicated by the center line12' of head 12 being exactly midway between the center lines 2a' of thetwo adjacent tracks. FIG. 3 shows that, when the upper head 12 is sopositioned, the lower head 14 will be exactly centered with one track(2b-2) on the lower face of the record disk, this condition beingindicated by the center line 14' of the head exactly coinciding with thecenter line 2b' of the respective track.

Thus, when recording on the lower face 2b of the disk, head 12 may beused in cooperation with the tracks on upper face 2a of the disk toexactly align head 14 with respect to a selected track on the lower faceof the disk, and once this is done, head 14 may then be used forrecording, reproducing or erasing the information with respect to theselected track. In a similar manner, head 14 cooperable with the lowerface of the disk may be used for centering head 12 on a selected trackon the upper face of the disk.

As one example, disk 2 may have tracks written on both faces at adensity of 200 tracks per inch, whereby the track center lines 2a', 2b'are spaced from each other 5 mils. In such a case, the center line 12'of the upper head 12 would be displaced 2.5 mils from the center line14' of the lower head 14, so that when the center line 12' of the upperhead is precisely positioned between two adjacent tracks on the upperface, the lower head center line 14' will be precisely positioned overthe center line of a track on the lower face, for recording,reproducing, or erasing data with respect thereto.

Thus, all the tracks on both faces of the magnetic disk are used forrecording informational data, each head cooperating with the magnetictracks on its respective face of the disk for positioning the other headto record or extract the informational data from the tracks on the otherface of the disk.

The manner of positioning one head precisely between two adjacent tracksis based on balancing the signal amplitudes from the two tracks, and isdescribed more particularly below with respect to FIG. 5.

Clock Frequencies of Recording

Since the data itself on the tracks is also used for positioningpurposes, the data on adjacent tracks are recorded at different clockfrequencies so that the data on one track can be discriminated from thedata on the other. A preferred scheme of clock frequencies is describedbelow. The system further includes filters for separating the differentfrequencies read by the positioning head before their amplitudes arecompared and used for controlling the transducer drive in order toposition the other head for data recording or reproducing purposes.

In many systems in which the disk file described herein is to be used,the data enters and leaves the disk file asynchronously, so that a totalfrequency difference of the order of 5% is completely transparent to thedata processing system. On the other hand, disk speed variations due tomotor-cogging, belt-slip, and the like, may be as high as 1%, with anadditional 1% for long-term variations.

The present invention provides an arrangement in which the clockfrequencies between adjacent tracks differ by at least 2%, with themaximum difference between the clock frequency of any particular trackand the nominal clock frequency being less than 5%. Thus, the system candiscriminate between the different clock frequencies of adjacent trackswithout special interfacing to the input or output units.

Briefly, this is accomplished by grouping the tracks into a plurality ofgroups with correspondingly-numbered tracks in each group being recordedat the same clock frequency. The different frequencies at which thetracks are recorded are interlaced between adjacent tracks such thateach odd track has a relatively large frequency difference with respectto the adjacent even track, but a relatively small frequency differencewith respect to the next odd track. Further, each group of tracks isdivided into two sub-groups, there being means for reversing thefrequency variations of each sub-group so that one sub-group decreasesin frequency with respect to a nominal frequency, and the othersub-groups increase in frequency with respect to the nominal frequency.

The foregoing arrangement is more particularly illustrated in thediagram of FIG. 4, wherein each group includes 16 tracks (T₁ -T₁₆)divided into two sub-groups (T₁ -T₈ and T₉ -T₁₆) of 8 tracks each. Thesystem uses 13 different clock frequencies.

Thus, as shown in FIG. 4, the center or nominal clock frequency is 3 MHzas represented by line NF. The lowermost clock frequency at which thedata signals are recorded is represented by line LF and differs from thenominal frequency NF by about 3.2%, and the uppermost frequency at whichthe data signals are recorded is represented by line UF and also differsfrom the nominal clock frequency NF by about 2.85%. It will thus be seenthat each square in the diagram of FIG. 4 represents a frequencydifference of approximately 0.2%.

The first odd track (T₁) receives the data signals at a clock frequencyof about 0.6% above the nominal clock frequency (line NF), and the nextadjacent even track (T₂) receives the data signals at a clock frequencyof about 2% below the nominal clock frequency. The frequency differencebetween the two adjacent tracks is thus about 2.6%.

The next odd track (T₃) receives the data signals at a clock frequencyof about 0.2% above the nominal clock frequency, while the next adjacenteven track (T₄) receives the data signals at a clock frequency of about2.4% below the nominal frequency, resulting in a frequency differencealso of about 2.6%.

The next adjacent track (T₅) is recorded at a clock frequency of about0.2% below the nominal frequency, while the next adjacent even track(T₆) is recorded at a clock frequency of about 2.8% below the nominalfrequency.

The last odd track (T₇) in the first sub-group records at a clockfrequency of about 0.6% below the nominal frequency, and the last eventrack (T₈ ) in the first sub-group records at a clock frequency of about3.2% below the nominal frequency.

In the first sub-group of track T₁ -T₈, therefore, while the frequencydifference between adjacent tracks is about 2.6% in each case, thegreatest deviation from the nominal frequency is represented by the lasteven track (T₈), and this deviation is only about 3.2% from the nominalfrequency.

FIG. 4 also illustrates the centre frequencies between two adjacenttracks: Point C₁,2 is the centre frequency between tracks T₁ and T₂ ;point C₂,3 is the centre frequency between tracks T₂ and T₃ ; point C₃,4is the centre frequency between tracks T₃, T₄ ; and so on.

It will be seen that in the first sub-group of tracks T₁ -T₈, the oddtracks are recorded at the higher clock frequency than their respectiveeven tracks; also, the clock frequencies of both odd and even trackscontinuously decrease with respect to the nominal frequency.

In the second sub-group of track T₉ -T₁₆, a phase reversal occurs sothat the clock frequencies of the tracks continuously increase withrespect to the nominal frequency; also, the odd tracks are recorded at alower clock frequency than their respective even tracks.

Thus, the first odd track (T₉) in the second sub-group is recorded at aclock frequency of about 1.0% below the nominal frequency, whereas itsadjacent even track (T₁₀) is recorded at a clock frequency of about 1.6%above the nominal frequency. The next adjacent odd track (T₁₁) isrecorded at about 0.6% below the nominal frequency, whereas its nextadjacent even track (T₁₂) is recorded at about 2.0% above the nominalfrequency. The next odd track (T₁₃) is recorded at about 0.2% below thenominal frequency, whereas its adjacent even track (T₁₄) is recorded atabout 2.4% above the nominal frequency; and the last odd track (T₁₅) isrecorded at about 0.2% above the nominal frequency whereas the last eventrack (T₁₆) is recorded at about 2.8% above the nominal track frequency.

The centre frequencies between adjacent tracks in the second sub-groupare also indicated in FIG. 4 by points C₈,9 ; C₉,10 ; C₁₀,11 ; C₁₁,12 ;C₁₂,13 ; C₁₃,14 ; C₁₄,15 ; and C₁₅,16.

It will thus be seen that also in the second sub-group, the frequenciesof adjacent tracks differ by about 2.6% with the maximum difference ofany track in the sub-group with respect to the nominal frequency beingabout 2.8%.

FIG. 4 illustrates these clock frequency differences in an approximatemanner for the sake of convenience. Optimumly, however, the frequencydifference between adjacent tracks should be about 2.8%, and the maximumdifference between the nominal frequency of any particular track shouldbe about 3.15%.

Overall System of FIG. 5

FIG. 5 illustrates the overall system that may be used for recording andreproducing the data at the different clock frequencies as discussedabove so as to enable one magnetic head to be used for positioningpurposes, and the other magnetic head to be used for actually recording,reproducing or erasing the data. The system of FIG. 5 also includesmeans for initializing a virgin magnetic disk, the specific procedureused for that purpose being described more fully below.

In FIG. 5 similar reference numbers are used for corresponding elementsillustrated in FIG. 1. Thus, the head position drive 16 is shown in FIG.5 as a linear actuator which is adapted to move the transducer headassembly 8 in a radial direction towards or away from spindle 4, inorder to properly position the upper and lower magnetic heads 12, 14with respect to the magnetic tracks on the upper and lower faces of themagnetic disk 2.

The central processor 18 (or other input device) feeds, via line 30, theinformational data to data memory or buffer 32, which stores the databefore being recorded on the magnetic disk 2. Central processor 18 alsosupplies the positional demand data, namely the address of the track onwhich the data is to be recorded, this data being fed via line 34 to aposition demand memory 36. Central processor 18 has a further outputline 38 controlling the initialization of a virgin record disk, as willbe described more fully below.

Substantially the remainder of the system illustrated in FIG. 5 can beconsidered within box 20 of FIG. 1 and concerns those elements which areinvolved in the data recording and reproducing operations, and also inthe positioning of the transducer head assembly to the selected magnetictrack of the record disk at which such operations are to be performed.

The data from memory device 32 is fed to a clocking network, generallydesignated 40, and is clocked at different frequencies (there being 13frequencies, F₁, F₂ . . . F₁₃, in the example illustrated) for recordingthe data on different tracks, as described above with reference to FIG.4. The specific clock frequency is determined by the address of thetrack on which the data is to be recorded, and is controlled by theposition demand memory 36 via line 42. This data, at the appropriateclock frequency, is then fed into a head change-over switch 44. Thelatter transmits the data via Read/Write amplifiers 46, 48 to the tworecording heads 12, 14, and also transmits the data from such heads viathe same amplifiers.

A signal from the position demand memory 36 is supplied via line 50 tohead change-over switch 44 and determines whether the data isinformational data to be supplied to the conventional data channel 52,or positional data, in which case it is supplied to the servo datachannel 54.

As explained earlier, the informational data is also used forhead-positioning purposes. Thus, the data is used as positional datawhen it is read by the positional head (e.g. head 12 in FIG. 2) disposedbetween two tracks on one face of the disk and outputted via servochannel 54, in order to exactly position the other head (e.g. 14, FIG.3) in alignment with a track on the other face of the disk in which thedata is to be recorded or reproduced. The data read from the latter headis outputted via informational data channel 52.

The data from the positional head is fed via line 54 to a filteringnetwork 56 which includes a plurality of filters, one for each of thefrequencies F₁ . . . F₁₃. The specific filters (two) to be madeeffective depend on the data tracks spanned by the positional head, andthis is controlled by the position demand memory 36 via line 58.Filtering network 56 thus provides two outputs, on output lines 60 and62, which correspond to the amplitudes of the signals read by thepositional head from the two adjacent tracks which it spans.

These amplitude signals are fed to a network 64 which produces ananalogue error signal corresponding to the difference in the amplitudesof the two input signals on lines 60 and 62. If the centre-line of thepositional magnetic head is disposed exactly midway between the twoadjacent tracks, the output of network 64 would be zero, whereas in allother cases the output would be of a value and sign depending on theamount and direction the head is off-position with respect to thecentre-line between the two adjacent tracks.

The output of amplitude difference network 64 is fed to a network 66which digitally controls the inversion of the amplitude differencesignal, this control being effected via line 68 by the position demandmemory 36. The output of network 66 is then fed to a summing network 70,the output of which is fed via servo amplifier 72 to the linear actuator16 which drives the transducer head assembly 8 with respect to themagnetic record disk 2.

Summing network 70 is also fed with the output of an amplifier 74, inaddition to the analogue signal corresponding to the difference inamplitudes of the signals on lines 60 and 62. The latter amplifier has,at one input 76, an analogue voltage corresponding to the demandedposition supplied by demand memory 36 via digital-to-analogue converter78. The second input to amplifier 74 is from a linear potentiometer 80for the coarse position control.

Network 66 may or may not invert the error signal from the amplitudedifference network 64, depending on whether the track being centered isan odd or even track.

Thus, if the track being centered is to correspond to center point C₁,2in FIG. 4, an increase in the amplitude of the higher frequency signal,produced by a forward error, will cause the servo to move the headassembly backwardly, thereby reducing the error to zero, at which timethe data head is exactly centered at point C₁,2.

On the other hand, if the track being centered is to correspond tocenter point C₂,3 in FIG. 4, an increase in the amplitude of the higherfrequency signal is to produce a forward movement of the head assemblyto point C₂,3, rather than a backward movement to point C₁,2. In such acase, a signal is supplied to network 66 via line 68 from the positiondemand memory 36, commanding network 66 to invert the error signal fromamplitude difference network 64, whereby the head assembly will be movedforwardly to point C₂,3.

As will be recalled, the arrangement described above with respect toFIG. 4 repeats the same clock frequency pattern with each group of 16tracks. Thus, there is a possible frequency ambiguity every 16 tracks.This can easily be overcome, however, by the coarse position controlsystem.

For this purpose, amplifier 74 is a switched-amplifier producing anoutput proportional to the difference in the inputs from line 76 and 82whenever such difference in input is greater than a predetermined numberless than 7, say three, tracks. When the difference is less than thepredetermined number, the output from amplifier 74 is zero, except wheninitializing as will be described below. Amplifier 74 thus providessufficient coarse position control to take care of the possibleambiguity occurring every 16 tracks.

Since the ambiguity has a cycle of 16 in the described system, 7 is themaximum error possible, as an error of over 8 brings it closer to thenext, ambiguous cycle, whereas an error exactly of 8 is unstable, bothcorrect and ambiguous settling points being equally likely. An error of3 in the above operation of the switched-amplifier 74 is thereforearbitrary, as the error could be up to 7 in the described system.

Any suitable known arrangement may be used for the clocking network toprovide the different clock frequencies for the various tracks. Onetechnique would be to use a disk with some 200 equally-spaced holes ortransparent sectors, and a conventional light chopper to provide a pulsestring which controls a phase-locked loop. Thus, the clock frequencywould be defined by the disk rotational speed. By including avariable-module counter in the loop, which is a well-known technique,the output frequency can be modified according to the clock frequency ofthe respective track as described above with reference to FIG. 4.

Initializing a Virgin Record Disk

Virgin disks may be initialized by using one drive to write a pluralityof disks with the initializing pattern of a master disk.

As indicated earlier, the system illustrated in FIG. 5 can also be usedfor initializing a virgin record disk. When the disk is to beinitialized, a signal is fed by the central processor 18 via line 38 toamplifier 74 to disable its track-ambiguity-correcting functiondescribed earlier. In such a case, the output of amplifier 74 would bethe sole control of the linear actuator 16 so that the latter wouldrespond only to the position demand signal supplied by central processor18 via line 76, and the course position potentiometer 80

which is used as the position transducer during this operation. Severalprocedures may be used for initializing a virgin record disk.

One procedure would be to record a plurality (e.g. all) the annular,spaced tracks on one face of the record disk by the use of one head(e.g. the upper head 12) of the transducer assembly; record a pluralityof the tracks on the other face of the record disk by positioning theupper head 12 exactly between adjacent tracks on the upper face of thedisk by the coarse and fine position control system of FIG. 5, whileusing the other head (e.g. the lower head 14) for recording the trackson a lower face of the disk; re-recording the tracks on the upper faceof the disk, with a reduced positional error between tracks, bypositioning the lower head of the assembly exactly between two adjacenttracks on the lower face, while using the upper head for re-recordingthe tracks on the upper face; and repeating the process on both faces ofthe record disk, each time further reducing the positional error betweentracks, until the positional error is reduced to the desired value.

For example, in a record disk having about 400 tracks on each face, witha track density of 200 tracks per inch and with a track width of 5mils., pitch errors can be reduced from about 2.5 mils to less than 0.5mils by rewriting the tracks about 3 times. It will be appreciated thatadditional rewriting would further reduce pitch errors.

Another technique that could be used for initializing a virgin recorddisk would be as follows: (1) write track "0" first with the upper headand then with the lower head; (2) write track "1" with the upper head.Using an arbitrary D.C. input to balance against the lower head output(with appropriate choice of D.C. input, this will be within ±1/4 trackpitch of the correct position for track "1"); (3) with the lower head,write track "1" using the upper head for positioning between track "0"and "1" ; (4 ) rewrite track "1" with the upper head, using track 0 and1 for positioning the lower head; (5) if necessary, repeat steps 3 and4; (6) measure the amplitude of the positional signal, resetting theD.C. bias input to the appropriate level alternately for each surface;(7) use these values to step forward in accordance with step 2 above fora small number of tracks; (8) repeat steps 3-6; and (9) continue theprocedure until all tracks are written with the initializing pattern.

While the invention has been described with reference to a preferredembodiment, it will be appreciated that the invention, or variousaspects thereof, could advantageously be used in other forms or in otherapplications.

What is claimed is:
 1. apparatus for use in recording, reproducing orerasing data, comprising:a record disk having a plurality ofequally-spaced, radial, recording tracks on both faces thereof, trackson one side being correspondingly, equal-radially spaced with respect torespective tracks on the other side, and variable informational databeing recorded onto said tracks at different clock frequencies withrespect to adjacent tracks; support means; an upper transducer headcarried by said support means and cooperable with the upper face of saiddisk for reading or writing informational data from or onto said tracks;a lower transducer head carried by said support means and cooperablewith the lower face of said disk for reading or writing informationaldata from or onto said tracks, said lower transducer head fixed in aradially spaced relationship from said upper head with respect to saiddisk by one-half the distance between pairs of adjacent of said equallyspaced tracks; and means for centering either one of said heads with asaid track by correspondingly centering other of said heads between twoadjacent of said tracks on the opposite face of said disk, said meanscomprising:1. means for separating reproduced data from said other headaccording to said different clock frequencies of said two adjacenttracks;
 2. Means for comparing the amplitude of said separatedreproduced data; and
 3. Drive means responsive to said comparing meansfor driving said heads radially with respect to said disk formaintaining equal amplitude comparison by said amplitude comparisonmeans.
 2. Apparatus according to claim 1, wherein, said transducer headsare magnetic heads; and said record disc is a magnetic record disc. 3.Apparatus according to claim 1, and further including:means foraddressing a data track for recording informational data thereon; andmeans responsive to said addressing means for clocking the informationaldata to be recorded on said addressed track at the respective clockfrequency associated with the addressed data track.
 4. Apparatusaccording to claim 1, and further including means for addressing a datatrack for reading or writing data thereon; and wherein said separatingmeans includes a plurality of frequency filter means, each of saidfilter means being frequency associated with a clock frequency of saidtracks; and wherein said separating means is responsive to saidaddressing means for affecting the said frequency filtering meanscorresponding to the clock frequency of the data tracks spanned by saidother head when said one head is centered with said addressed track. 5.Apparatus according to claim 1, and further including:means foraddressing a data track for reading/writing data thereon; and switchmeans responsive to said addressing means for supplying said recordingdata on the said head associated with the disc side opposite saidaddressed track, to said separating means, and said switch means fortransmitting informational data from or to the other of said heads forreading or writing data from, or onto said tracks.
 6. Apparatusaccording to claim 5 wherein said drive means is responsive to saidaddress means for determining the direction of radial driving accordingto said addressed track.
 7. Apparatus according to claim 1, wherein saidtracks are grouped in a plurality of groups, each of said groupsincluding an equal plurality of tracks having corresponding tracks ineach group recorded at the same clock frequency.
 8. Apparatus accordingto claim 7, wherein said different clock frequencies at which saidtracks are recorded are interlaced in relatively large clock frequencydifferences between adjacent tracks and relatively small clock frequencydifferences between alternate adjacent tracks.
 9. Apparatus according toclaim 8, wherein said clock frequencies of adjacent tracks differ by atleast 2% and the maximum difference between a nominal clock frequencyand the clock frequency of any one track is less than 5%.
 10. Apparatusaccording to claim 8, wherein each group of said tracks includes twosub-groups, the clock frequency centers between adjacent tracksdecreasing in clock frequency with respect to a nominal clock frequencyin one of said sub-groups, and increasing in clock frequency withrespect to the nominal clock frequency in the other of said sub-groups.11. Apparatus according to claim 10, wherein said clock frequencies ofadjacent tracks differ by at least 2% and the maximum difference betweenthe nominal clock frequency and the clock frequency of any one track isless than 5%.